Coupled quantum well devices (CQWD), such as those described, for example, in A. Palevski, et al. “Resistance Resonance in Coupled Potential Wells,” Physical Review Letters, Vol. 65, No. 15, pg. 1929 (1990), Y. Katayama, et al. “Lumped Circuit Model of Two-Dimensional Tunneling Transistors,” Appl. Phys. Lett. Vol. 62, No. 20, pg. 2563 (1993), J. A. Simmons, et al. “Unipolar Complementary Bistable Memories Using Gate-Controlled Negative Differential Resistance in a 2D-2D Quantum Tunneling Transistor,” International Electron Devices Meeting, 1997, Technical Digest, Dec. 7-10, 1997, pgs. 755-758, J. P. Eisenstein, et al. “Independently Contracted Two-Dimensional Electron Systems in Double Quantum Wells,” Appl. Phys. Lett. Vol. 57, No. 26, p. 2324 (1990), and I. B. Spielman, et al. “Observation of a Linearly Dispersing Collective Mode in a Quantum Hall Ferromagnet,” Physical Review Letters, Vol. 87, No. 3, (2001), are tunneling devices that can operate at very low voltages (e.g., 0.1V to 0.3V), which yield major advantages in terms of power consumption and are interesting to investigate in order to evaluate their potential.
In its simplest, two-layer form, a CQWD contains two quantum well layers that are separated by a thin barrier layer. Each quantum well layer includes a very thin sheet of 2-dimensional electron gas (2DEG) that lies in the xy plane. The two 2DEG sheets are superimposed at different positions along the z axis, with a narrow tunneling gap between them. When the energy levels in these two 2DEG sheets are degenerate (i.e., their quantum states share the same quantum numbers), they are coupled strongly due to the inter-sheet tunneling effect. When the energy levels are not equal, tunneling coupling falls off, and the energy difference becomes comparable with the inter-sheet matrix element, forming a simple switching device. For more details relating to the two-layer CQWD structure, see the articles by Palevski, et al., Katayama, et al and J. A. Simmons, et al. as mentioned hereinabove.
Three-layer CQWD structures have been shown to constitute a latch, which can form the basis of logic and memory circuitry. This is described, for example, in U.S. Pat. No. 5,625,589 issued to Katayama on Apr. 29, 1997 for “Static Memory Cell with Spaced Apart Conducting Layers,” and Y. Katayama “New Complementary Logic Circuits using Coupled Quantum Wells,” IEEE Nano (2004).
CQWDs, which use quantization of single particle states locally, constitute a preliminary step towards computing with quantum systems.
Further, it has been discovered that fundamental physics is involved at low temperature in CQWDs under appropriate conditions (see, the articles to I. B. Spielman, et al. and Eisenstein, et al. mentioned above), and this will be another promising field for future investigation.
However, in the prior art CQWD structure, the multiple quantum well layers are typically connected in parallel, i.e., by the same set of electrodes that simultaneously contact all the quantum well layers, due to difficulties in making selective contact to the individual quantum wells. The quantum wells are either operated in parallel, or series operation can be indirectly achieved by using additional top and bottom turn-off electrodes.
For example, FIG. 1A illustrates a prior art CQWD structure 10 with two quantum well layers 12 and 14, which are spaced apart by a very thin barrier layer 16. Suitable doping is provided to establish a 2DEG sheet in each well. Standard diffused-in contacts 18 and 20 are provided, each of which contacts both quantum well layers 12 and 14 simultaneously. Additional turn-off electrodes 22 and 24 are respectively fabricated on the top and bottom surfaces of the CQWD structure. Application of a negative bias potential Vt to the top turn-off electrode 22 will result in depletion of the electrons within the quantum well regions that are underneath the electrode 22. For small −Vt, the depletion only occurs within a region in the upper quantum well layer 12. Continued increase of −Vt can eventually fully deplete the upper quantum well layer 12 and begin depletion of the lower quantum well layer 14. Therefore, a range of top turn-off bias voltages exists in which the two ohmic contacts 18 and 20 are electrically connected together only via the lower quantum well layer 14. In exact analogy, the bottom turn-off electrode 24 can be negatively biased by a voltage −Vb, so that the lower quantum well layer 14 above it is fully depleted but the upper quantum well layer 12 is not. In such a manner, the top and bottom turn-off electrodes 22 and 24 can be suitably biased to provide a condition in which the contacts 18 and 20 are electrically disconnected from each other by the depleted regions in the upper and lower quantum well layers 12 and 14, as shown in FIG. 1B. When the energy levels in the upper and lower quantum well layers 12 and 14 are degenerate, electrons can flow from contact 18 to contact 20, or vise versa, through un-depleted regions in the quantum well layers 12 and 14 and across the barrier layer 16 via tunneling effect, as indicated by the arrowheads in FIG. 1B.
In the prior art CQWD structure illustrated by FIGS. 1A and 1B, the contacts 18 and 20 are not selectively contacted with the individual quantum well layers 12 and 14. Instead, they are simultaneously contacted with both quantum wells, and the top and bottom turn-off electrodes 22 and 24 must be employed to selectively deplete regions in the quantum well layers 12 and 14, in order to indirectly establish selective electrical connection between contacts 18 and 20 and quantum well layers 14 and 12, respectively.
The top and bottom turn-off electrodes of the prior art CQWD are functionally and structurally clumsy. Further, they are limited to 2-layer structures, which is not suitable for computer applications.